This invention relates to a method and apparatus for fuel control of an internal combustion engine for a motor vehicle and, more particularly, to a method and apparatus for fuel control, which are capable of supplying an engine with fuel of a suitable amount when an operational condition of the engine has been changed from a low-speed operational condition to a suddenly-accelerated condition such as a fully opened throttle valve.
In general, a flow rate of air flowing into an engine varies in proportion to an opening degree of a throttle valve. However, when the throttle valve, in a fully-closed state, is operated to fully open state, the air flow does not respond since the air suction passage has a length from the engine to the throttle valve and an air flow rate sensor is provided on the upstream side of the throttle valve. When the throttle valve is moved in an opening direction thereof, the engine is accelerated, and the A/F (air-fuel) ratio must be reduced. However, due to the above arrangement, the air flow passing at the air flow rate sensor has not reached an air flow rate corresponding to the throttle opening as yet. Therefore, when a flow rate of sucked or drawn in air is detected by the air flow rate sensor, an optimum fuel supply amount is calculated based on this flow rate and the fuel is ejected by an injector. Therefore, the A/F (air-fuel) ratio increases (fuel is lean), and the engine is not sufficiently accelerated. In order to eliminate this inconvenience, a method of correcting the control delay has been employed, in which the fuel feed rate is determined by the air flow rate sensor in accordance with the degree of opening of the throttle valve.
In the conventional acceleration correcting system using a throttle sensor disclosed in Japanese patent laid-open No. 185949/1983, the so-called fuel increment correction for acceleration is carried out, in which, when the quantity of variation per predetermined period of time, i.e. differentiation quantity, of an output from the throttle sensor is detected and the quantity of variation of the throttle sensor output exceeds a predetermined level, the fuel feed rate which is computed based on the air suction rate detected by the air flow rate sensor is multiplied by a certain coefficient, for example, 1.1, to increase the fuel feed rate.
However, the conventional acceleration correction system has the following drawbacks. Namely, when the engine is suddenly accelerated to such an extent that the throttle valve is fully opened while the engine is in a low-speed operation, for example, at 800-1000 rpm, the air suction rate increases in accordance with the increase in the degree of opening of the throttle valve but the fuel feed rate does not sufficiently increase in spite of increase of fuel for the acceleration because the fuel is deposited on the inner surface of the manifold. Consequently, desired acceleration characteristics cannot be obtained. If the fuel increase for acceleration is increased on every occasion when the engine is accelerated so as to eliminate these inconveniences, the mixing ratio of the fuel in an operational range other than the fuel injection rate increasing operational range, a so-called power zone increases, so that emissions in the exhaust gas are diminished.